Diffusers scatter incident electromagnetic radiation (e.g., visible light, infrared, and ultraviolet radiation) by means of diffuse transmission or reflectance. Considered as components of imaging and non-imaging optical systems design, an ideal diffuser would exhibit the following physical characteristics:
1. Scattering within a specified beam distribution. When a ray of collimated (but not necessarily coherent) light is incident upon a diffuser at an angle θ(i), the transmitted or reflected light would be randomly scattered through a range of angles between θ(t1) and θ(t2) for transmitted light or θ(r1) and θ(r2) for reflected light. These angles are shown in FIG. 1 and represent the limits of the transmitted or reflected beam distribution pattern. FIG. 1 is a schematic illustration of the scattering of incident beams of collimated light by prior art transmissive and reflective diffusers. Collimated light beam 10 is perpendicular to the surface of a conventional transmissive diffuser 12 and is scattered into a beam distribution 14. The beam distribution maximum is perpendicular to the surface of diffuser 12. A collimated light beam 16 is incident to a surface normal n of conventional diffuser 12 at an angle θ(i) and is scattered into a beam distribution 18. The beam distribution maximum (the “central axis” of the beam distribution) is inclined at an angle θ(t) relative to surface normal n of diffuser 12 and is equal to angle θ(i).
A collimated light beam 20 is perpendicular to the surface of a conventional reflective diffuser 22 and is scattered into a beam distribution 24. The beam distribution maximum is perpendicular to the surface of diffuser 22. A collimated light beam 26 is incident to a surface normal n of conventional diffuser 22 at an angle θ(i) and is scattered into a beam distribution 28. The beam distribution maximum is inclined at an angle θ(r) relative to surface normal n of diffuser 22 and is equal to angle θ(i).
2. No scattering outside of the specified beam distribution. No incident light would be scattered outside of the specified beam distribution ranges.
3. Uniform beam distribution. The incident light would be uniformly scattered within the specified beam distribution.
4. No backscatter. If the diffuser transmits rather than reflects incident light, none of the incident light would be reflected by the diffuser.
5. No absorption. None of the incident light would be absorbed by a transmissive diffuser.
6. Complete diffusion. It would not be possible to see an image of the light source or “hot spot” when looking at the light source through a transmissive diffuser. The diffuser would appear to have a constant luminance (“photometric brightness”) distribution across its surface.
7. Wavelength independence. The scattering properties of the diffuser would be independent of the wavelength of the incident light over a specified range of wavelengths.
For the purposes of optical systems design flexibility, two additional physical characteristics would sometimes be desirable:
8. Anisotropic beam distribution. The beam distribution of the diffuser would be anisotropic about the central beam axis, including distributions that are elliptical or linear, as shown in FIG. 2. FIG. 2 is a schematic illustration of the cross-sections of the scattered light beam distributions from a prior art isotropic (circular) diffuser 30, elliptical anisotropic diffuser 32, and substantially linear anisotropic diffuser 34.
9. Off-axis beam distribution. The central axis of the beam distribution would be at a transmitted angle, θ(t) or θ(t′), as shown in FIGS. 3A and 3B,respectively, that is not equal to the incidence angle, θ(i), as might be predicted by Snell's Law. The beam distribution also varies depending on the pattern orientation. In particular, batwing distributions (FIGS. 8C and 8D) are achievable with the pattern facing the light source. Similarly, the central axis of the beam distribution would be at a reflected angle, θ(r), as shown in FIG. 4, that is not equal to the incidence angle, θ(i), as might be predicted by the law of reflection from specular surfaces. FIG. 4 is a schematic illustration of an off-axis beam distribution for a reflective kinoform diffuser designed in accordance with the invention. A collimated beam 42 is incident to a surface with normal n of a reflective kinoform diffuser 44 at an angle θ(i) and is scattered into a beam distribution 46. The beam distribution maximum is at an angle θ(r) to surface normal n of kinoform diffuser 44, where angle θ(r) is generally not equal to angle θ(i)
Kinoform diffusers may exhibit certain physical characteristics that approach those of an ideal diffuser.
Lesem, L. B., Hirsch, P. M., and Jordan, Jr., J. A., “The Kinoform: A New Wavefront Reconstruction Device,” IBM J. Research and Development, 13:150-55(1969) introduced a “kinoform,” describing it as a computer-generated “wavefront reconstruction device” that, similar to a hologram, provides the display of a three-dimensional image. Unlike a hologram, however, the kinoform yields a single diffraction order in which all of the incident light is used to reconstruct the image. A kinoform operates only on the phase of an incident wave, because it is assumed that only the phase information in a scattered wavefront is required for the construction of an image of the scattering object. The amplitude of the wavefront in the kinoform plane is assumed to be constant.
Caulfield, H. J., “Kinoform Diffusers,” SPIE Vol. 25, Developments in Holography, 111-13 (1971) stated that a kinoform of a “scattering object” constituting a conventional diffuser, such as ground glass, could be generated by photographic techniques, thereby producing a “kinoform diffuser.” U.S. Pat. No. 3,619,021 of Biedermann et al. describes a technique for constructing a kinoform diffuser, which is called in their patent simply a “diffusing layer.”
FIG. 5 shows a basic prior art optical setup used to record kinoform diffusers as taught by Caulfield. (As will be appreciated by those skilled in the holographic arts, many variations in the optical setup are possible.) A laser 48 produces a beam of coherent light 50 that is expanded by lenses 52 and 54 to fully and evenly illuminate a diffuser 60 with a coherent planar wavefront propagating through an opaque mask 56 having an aperture 58. A photosensitive recording plate 62 is located a distance, d, behind diffuser 60. (Suitable photosensitive materials include positive and negative photoresist emulsions, silver halide films, dichromated gelatin, and various photopolymers.)
The light scattered by diffuser 60 produces on a surface of photosensitive recording plate 62 a random laser speckle pattern that is recorded photographically. Photosensitive plate 62 is developed in accordance with known processing techniques to produce a transparent substrate with a surface relief pattern whose spatially distributed height distribution is proportional to the spatially distributed intensity of the recorded laser speckle pattern, which is shown in FIG. 6. This is the transmissive kinoform diffuser. A reflective kinoform diffuser can be fabricated by, for example, applying an evaporated metal film to the surface of the transmissive diffuser. Alternatively, the surface relief pattern can be transferred using known replication techniques such as embossing or molding to an opaque metallic or plastic substrate.
When the transmissive kinoform diffuser is illuminated by a coherent planar wavefront, the length of the optical path through the diffuser at any point is determined by the height of the surface relief pattern at that point. Because the phase retardation of the wavefront propagating through the diffuser is dependent on the optical path length, the planar wavefront is randomly scattered according to the surface relief pattern of the kinoform diffuser. In theory, the kinoform diffuser reconstructs the laser speckle pattern generated by ground glass diffuser 60.
The same principle applies to reflective kinoform diffusers, except that the differences in optical path length and subsequent phase retardation occur in free air or other optically transparent medium immediately above the diffuser surface.
The Caulfield publication and certain other references noted the following observations:
1. The beam distribution of the kinoform diffuser is dependent on the distance, d, between diffuser 60 and recording plate 62. Increasing d decreases the range of angles θ(t1) to θ(t2), between which substantial diffusion occurs.
2. The angular intensity distribution of scattering is highly nonuniform, as shown in FIG. 7. Dainty, J. C., “The Statistics of Speckle Patterns,” Progress in Optics XIV, E. Wolf, (ed.), New York, N.Y.: North-Holland, 346 (1976) sets forth the following expression demonstrating that the expected beam distribution can be characterized by a negative exponential function:Iθ=A*exp(−B*I0),  (1)where Iθ is the expected intensity at angle θ from the axis of the incident ray, I0 is the incident ray intensity, and A and B are positive constants.
3. Tilting diffuser 60 or recording plate 62 about an axis perpendicular to the laser beam axis produces kinoform diffusers with anisotropic beam distributions that are approximately elliptical, as shown in FIG. 2. Wadle S., Wuest, D., Cantalupo, J., et al., “Holographic Diffusers,” Optical Engineering, 33(1):213-18 (January 1994) stated that the equivalent effect can be achieved by using a rectangular aperture in opaque mask 56, and U.S. Pat. No. 3,698,810 of Bestenreiner et al. described the use of one narrow slit aperture or multiple narrow slit apertures to produce substantially linear beam distributions.
4. Gray, P. F., “A Method for Forming Optical Diffusers of Simple Known Statistical Properties,” Optica Acta 25(8):765-775, noted that the expected beam distribution of kinoform diffusers produced using N multiple exposures with uncorrelated laser speckle patterns can be characterized by the function:P=(IN−1/(N−1)!)*exp(−N*I).  (2)
This function tends towards a substantially Gaussian function as the number of exposures N increases, as shown in FIG. 9.
The Lesem et al. publication noted that while there is only one image (i.e., diffraction order) formed in the laser speckle pattern reconstruction, there might be a “zero-order beam” component representing a portion of the undiffracted planar wavefront. Visually, the light source illuminating a kinoform diffuser can be seen when it is viewed directly through the diffuser, indicating incomplete diffusion. This blurred image can theoretically be eliminated by perfect phase matching within the kinoform.
The Caulfield publication demonstrated that elimination of the zero-order beam (and hence complete diffusion) could be achieved experimentally by adjusting the exposure of the photosensitive plate such that the transmitted beam was not visible through the kinoform diffuser. However, this applied only to monochromatic light sources. Kowalczyk, M., “Spectral and Imaging Properties of Uniform Diffusers,” J. Optical Society of America, A1(2):192-200 (February 1984) performed a theoretical analysis of kinoform diffusers and demonstrated that phase matching is wavelength-dependent. That is, the zero-order beam component can (in theory) be eliminated for monochromatic illumination only. When illuminated by an achromatic (or “white”) light source, these diffusers may exhibit significant spectral dispersion that appears as color bands surrounding the light source image that will be visible through the diffuser.
Although they were originally developed for holographic recording and reconstruction purposes, kinoform diffusers also effectively scatter quasi-monochromatic and polychromatic light, such as that produced by light-emitting diodes, and substantially achromatic light, such as daylight and artificial light produced by incandescent, fluorescent, and high intensity discharge lamps. Examples of such uses are given in U.S. Pat. No. 4,602,843 of Glaser-Inbari, U.S. Pat. No. 5,473,516 of Van Order et al., U.S. Pat. No. 5,534,386 of Petersen et al., and U.S. Pat. No. 5,701,015 of Lungershausen et al.
Kinoform diffusers for achromatic light applications of a type known as “surface-relief holographic diffusers” are commercially available. For example, Physical Optics Corporation (Torrance, Calif.) manufactures a series of products called “Light Shaping Diffusers.” These diffusers may exhibit substantial elimination of the zero-order beam with achromatic light sources. That is, they are wavelength-independent across the visible spectrum. As taught by Gray, this can be achieved by exposing the photosensitive plate to a multiplicity of uncorrelated laser speckle patterns.
A disadvantage of surface-relief holographic diffusers is that their surface relief height distributions are (within the limits of known photographic recording techniques and replication technologies) directly proportional to the intensity distributions of the recorded laser speckle patterns. As shown theoretically by Dainty and experimentally by Gray, their beam distributions are necessarily characterized by substantially Gaussian functions.
A properly designed kinoform diffuser may, therefore, exhibit the following generally desirable physical characteristics:
1. Scattering within a specified beam distribution. The range of angles within which substantial scattering occurs may be controlled by varying the distance, d, between diffuser 60 and recording plate 62 (FIG. 5).
2. Minimal backscatter. Backscatter may occur substantially only by reflection from the surfaces of a transmissive kinoform diffuser and may be substantially eliminated by the application of suitable antireflection coatings to said surfaces.
3. Minimal absorption. Incident light is absorbed substantially only within the transparent substrate of a transmissive kinoform diffuser.
4. Anisotropic beam distribution. The eccentricity of an elliptical beam distribution may be determined by the ratio of length to width of rectangular aperture 58 in opaque mask 56 (FIG. 5).
5. Complete diffusion. When it is purposefully designed to provide substantial elimination of the zero-order beam with achromatic light sources, the kinoform diffuser exhibits substantially complete diffusion of the incident light and freedom from spectral dispersion.
Unfortunately, a kinoform diffuser may also exhibit the following generally undesirable physical characteristics:
1. Significant scattering outside of the specified beam distribution. Because the expected beam distribution is characterized by a negative exponential or substantially Gaussian function, prior art techniques do not limit the scattering of the incident light to be fully within a specified range of angles.
2. Non-uniform beam distribution. A kinoform diffuser constructed using prior art techniques exhibits within the specified range of angles an expected beam distribution that is necessarily of a nonuniform negative exponential or substantially Gaussian shape. Kurtz, C. N., “Transmittance Characteristics of Surface Diffusers and the Design of Nearly Band-Limited Binary Diffusers,” J. Optical Society of America 62(8):982-989 (August 1972) and others show that kinoform diffusers with uniform beam distributions are theoretically possible, but provide no guidance in how they might be physically realized.
One preferred use of the kinoform diffusers of the present invention is their implementation in luminaires. Luminaires (also known as “light fixtures”) intended for general illumination applications are designed with the objectives of providing specific luminous intensity distributions while minimizing glare at high viewing angles and light losses within the luminaire housing. Designing luminaires to meet these objectives can be challenging, particularly when there are restraints on the physical size of the luminaire.
The luminous intensity distribution is determined by the placement and optical properties of lamps and light control components such as reflectors, refractors, diffusers, and shields (including louvers and baffles). There are many applications in which anisotropic luminous intensity distributions are used. For example, indirect fluorescent luminaires intended for office lighting typically require so-called “batwing” distributions (FIGS. 13A-13C) that provide even illumination of the ceiling (Illuminating Engineering Society of North America (IESNA) [2000]). The light control components are designed to redirect or absorb the light emitted by the lamps to achieve the desired luminous intensity distribution.
A disadvantage of light control components is that they absorb light and thereby reduce the luminaire efficiency. American National Standards Institute (ANSI)/IESNA [1996] defines luminaire efficiency as: “The ratio of the luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used therein.”) Tradeoffs are, therefore, made by a designer between the need to achieve specific luminous intensity distributions and minimum acceptable luminaire efficiencies.
Another aspect of luminaire design is the minimization of glare at high viewing angles (FIG. 14). (ANSI/IESNA [1996] defines glare as: “The sensation produced by luminances within the visual field of view that are sufficiently greater than the luminance to which the eyes are adapted to cause annoyance, discomfort, or loss in visual performance or visibility.”) For example, ceiling-mounted office luminaires 70 should direct most of their emitted light downward to the work plane 72. If they emit too much light horizontally, they will appear distractingly bright when viewed directly. Worse, their veiling reflections from computer monitor screens may reduce office productivity.
Glare can be minimized by blocking the emitted light with shields. However, this increases the light losses within the luminaire housing and so reduces the luminaire efficiency. These losses can be reduced by using reflectors or refractors instead of shields, but this approach may limit a designer's ability to achieve specific luminous intensity distributions.
Glare can also be minimized using glass or plastic diffusers. These are preferable to shields in that the light is emitted from a larger surface area (that is, the diffuser instead of the lamp) and so reduces the maximum luminance of the luminaire (IESNA [2000]). However, these diffusers typically absorb as much as one-half of the incident light, thereby reducing the luminaire efficiency. They also emit light in all directions within the hemisphere above their surfaces, thereby further limiting a designer's ability to achieve specific luminous intensity distributions.
In the related field of daylighting, light control devices such as shields and diffusers are often used to control sunlight entering a building through windows and skylights. Diffusers such as frosted glass and plastic panels are used to limit glare and reduce dark shadows, while light control devices such as louvers, mirrors, and motor-driven heliostats may be used to control and redirect sunlight through windows and skylights. As with luminaires, however, diffusers absorb a considerable portion of the incident sunlight and offer little control over the distribution of the diffused light.
There have been numerous prior attempts to control the luminous intensity distribution of luminaires and light sources using diffractive volume holograms and commercial holographic diffusers (which have similar optical performance characteristics to those of kinoform diffusers).
Davis (U.S. Pat. Nos. 4,536,833, 4,704,666, 4,713,738, and 4,722,037) described the use of multi-layered holograms as light control elements. Unlike kinoform diffusers, multi-layer holograms do not provide controllable diffusion or exhibit off-axis transmission properties, which are features of the present invention. They also function usefully as light control elements only for predetermined wavelengths. When used with achromatic light sources such as are commonly used for general illumination applications, multi-layer holograms exhibit unacceptable spectral dispersion effects (visible as color “fringes”) and high absorption characteristics.
Jannson et al. (U.S. Pat. No. 5,365,354) described various applications of volume holographic diffusers that involve luminaires designed for general illumination applications. However, these applications rely solely on the well-known anisotropic diffusion capabilities of commercial holographic diffusers.
Petersen et al. (U.S. Pat. No. 5,534,386) similarly described various applications of surface-relief holographic diffusers that involve luminaires designed for general illumination applications. These applications also rely solely on the anisotropic diffusion capabilities of holographic diffusers.
Van Order et al. (U.S. Pat. Nos. 5,473,516 and 5,582,474) described a vehicle light assembly that utilizes a holographic diffuser with circular or elliptical luminous intensity distribution characteristics. This vehicle light assembly requires that the light emitted from the lamp be substantially collimated by a reflector to effectively illuminate the holographic diffuser.
Fox (U.S. Pat. No. 5,630,661) described a metal arc flashlight that optionally includes a holographic diffuser. This flashlight also requires that the light emitted from the lamp be substantially collimated by a reflector to effectively illuminate the holographic diffuser.
Smith (U.S. Pat. No. 5,669,693) described an automotive tail lamp assembly that utilizes a holographic element to diffract light emitted by a light-emitting diode assembly in a preferred direction. This tail lamp assembly relies on the quasi-monochromatic emission of light-emitting diodes, and is not suitable for use with achromatic light sources such as incandescent or high-intensity discharge lamps.
Lungershausen et al. (U.S. Pat. No. 5,701,015) described an infrared illumination system for digital cameras that utilizes a holographic diffuser to homogenize the light emitted by infrared laser diodes. This illumination system requires that the emitted light be substantially collimated to effectively illuminate the holographic diffuser.
Hewitt (U.S. Pat. No. 6,062,710) described various luminaire designs that utilize holographic diffusers to reduce glare. Unlike the present invention, these designs are predicated on the use of imaging optical elements to substantially collimate the light that illuminates the holographic diffuser.
Shie et al. (WIPO International Publication Number WO 00/11498) described various applications of holographic diffusers that involve luminaires designed for general illumination applications. These applications are based on the process of molding surface-relief diffusers directly onto the surface of transparent optical elements using injection molding or casting. The described applications rely solely on the anisotropic diffusion capabilities of holographic diffusers and some mechanically produced diffusion patterns.
Shie et al. (WIPO International Publication Number WO 00/11522) further described various applications of holographic diffusers that involve luminaires designed for general illumination applications. These applications are based on the process of embossing surface-relief diffusers directly onto the surface of transparent optical elements using a sol gel process. The described applications similarly rely solely on the anisotropic diffusion capabilities of holographic diffusers and some mechanically produced diffusion patterns.
Saito (Japanese Patent No. 6-76618) described a lighting system comprising a light source and a holographic element acting as a dichroic mirror to reflect light of substantially one wavelength. The lighting system does not function properly when used with achromatic light sources.
Regarding daylight control, large plastic diffraction gratings have been used to redirect sunlight entering building through skylights and windows. The disadvantage of using such gratings is that they exhibit severe spectral dispersion. This is evident both as color fringes surrounding objects viewed through the gratings and as the separation of sunlight into a diffuse color spectrum that is visible on the walls, floor, and ceiling of the room.
Multi-layer volume holograms have been used as a replacement for diffraction gratings in an attempt to limit the effects of spectral dispersion. However, these light control devices suffer from low transmittance and consequent poor daylight utilization.